The invention relates to a process for the preparation of modified aerogels and to their use.
Aerogels, especially those with porosities above 60% and densities below 0.6 g/cm.sup.3, have an extremely low thermal conductivity and are therefore used as thermal insulation material, as described, for example, in EP-A-0 171 722.
Aerogels in the wider sense, i.e. in the sense of "gel with air as dispersion medium", are prepared by drying a suitable gel. The term "aerogel" in this sense includes aerogels in the narrower sense, xerogels and cryogels. A dried gel is described as an aerogel in the narrower sense if the gel liquid is removed at temperatures above the critical temperature and starting from pressures above the critical pressure. If, in contrast, the gel liquid is removed subcritically, for example with formation of a liquid-vapor boundary phase, then the resultant gel is described as a xerogel. It should be pointed out that the novel gels are aerogels in the sense of gels with air as dispersion medium. However, since these gels are prepared by subcritical drying, they could also be described as xerogels.
SiO.sub.2 aerogels may be prepared, for example, by acid hydrolysis of tetraethyl orthosilicate in ethanol. The hydrolysis gives a gel whose structure is determined, inter alia, by the temperature, the pH and the duration of the gelling process. However, the gel structure generally collapses when the wet gels are dried, since the capillary forces which arise during the drying are extremely large. The collapse of the gel can be inhibited by carrying out the drying above the critical temperature and the critical pressure of the solvent. Since the liquid/gaseous phase boundary disappears in this region, the capillary forces no longer apply and the gel does not undergo any change during the drying, i.e. there is also no shrinkage of the gel during the drying. Preparation processes based on this drying technique are known, for example, from EP-A-0 396 076 and WO 92/03378. However, this technique requires, for example if ethanol is used, a temperature of about 240.degree. C. and pressures of above 60 bar. If ethanol is exchanged for CO.sub.2 before the drying, the drying temperature is lowered to about 30.degree. C., but the required pressure is then above 70 bar.
An alternative to the abovementioned drying is given by a process for subcritical drying of SiO.sub.2 gels, if these are reacted with a chlorine-containing silylating agent before drying. The SiO.sub.2 gel here can be obtained, for example, by acid hydrolysis of tetraalkoxysilanes, preferably tetraethoxysilane (TEOS), using water in a suitable organic solvent, preferably ethanol. After exchanging the solvent for a suitable organic solvent, the gel obtained is reacted, in a further step, with a chlorine-containing silylating agent. Methylchlorosilanes (Me.sub.4-n SiCl.sub.n with n=from 1 to 3) are preferably used as silylating agent, because of their reactivity. The resultant SiO.sub.2 gel, which is modified on the surface with methylsilyl groups, can then be dried from an organic solvent in air. In this way, aerogels with densities below 0.4 g/cm.sup.3 and porosities above 60% can be attained. The preparation process based on this drying technique is described in detail in WO 94/25149.
To increase the strength of the gel structure, the above-described gels can also be mixed and aged with tetraalkoxysilanes in the alcoholic aqueous solution, before drying, as disclosed in WO 92/20623.
However, the tetraalkoxysilanes used as starting materials in the above-described processes are exceptionally costly. In addition, during the silylation with chlorine-containing silylating agents, large amounts of hydrogen chloride (HCl) and a wide variety of by-products associated with it are inevitably produced and these by-products may require a very complicated and costly purification of the silylated SiO.sub.2 gels by repeated washing with a suitable organic solvent. The especially corrosion-resistant production plants associated with this are likewise very expensive. The safety risk associated with the production of very large quantities of HCl gas, in addition, requires very complicated engineering and is therefore likewise very costly.
A first and not insignificant cost reduction can be achieved by using waterglass as starting material for the preparation of the SiO.sub.2 gels. For example, using an ion-exchange resin and starting with an aqueous waterglass solution, a silica can be prepared which polycondenses on addition of a base to give an SiO.sub.2 gel. After exchanging the aqueous medium for a suitable organic solvent, the gel obtained is then reacted, in a further step, with a chlorine-containing silylating agent. Methylchlorosilanes (Me.sub.4-n SiCl.sub.n with n=from 1 to 3) are likewise preferably used as silylating agent, because of their reactivity. The resultant SiO.sub.2 gel, which is modified on the surface with methylsilyl groups, can likewise then be dried from an organic solvent in air. The preparation process based on this technique is known from DE-A-43 42 548.
The problems, described earlier, of the exceptionally high process costs associated with the use of chlorine-containing silylating agents are, however, not solved by using waterglass as starting material.